ATC stands for Artist Trading Card. Basically, they are small works of art. The wonderful thing about ATC's is that there is only one "rule" - they must be 2.5 X 3.5 inches in size (vertical or horizontal) Other than that, you imagination can run wild. I was first introduced to ATC's in the "real" world, but they translate into the digi world just as well. Check out the gallery here: LINK or "Google" Artist Trading Cards to get an idea of the wide range of possibilities